linux-kernel - Re: [PATCH v5 0/7] bits: Fixed-type GENMASK()/BIT()

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Message-ID: <34c42bf9-a9e6-41db-9efb-a32bb253b4d2@wanadoo.fr>
Date: Thu, 6 Mar 2025 23:56:54 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Yury Norov <yury.norov@...il.com>,
 Lucas De Marchi <lucas.demarchi@...el.com>,
 Rasmus Villemoes <linux@...musvillemoes.dk>,
 Jani Nikula <jani.nikula@...ux.intel.com>,
 Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>, Tvrtko Ursulin
 <tursulin@...ulin.net>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Andrew Morton <akpm@...ux-foundation.org>,
 linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
 dri-devel@...ts.freedesktop.org, Andi Shyti <andi.shyti@...ux.intel.com>,
 David Laight <David.Laight@...lab.com>,
 Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
 Jani Nikula <jani.nikula@...el.com>
Subject: Re: [PATCH v5 0/7] bits: Fixed-type GENMASK()/BIT()

On 06/03/2025 at 22:02, Andy Shevchenko wrote:
> On Thu, Mar 06, 2025 at 08:29:51PM +0900, Vincent Mailhol via B4 Relay wrote:
>> Introduce some fixed width variant of the GENMASK() and the BIT()
>> macros in bits.h. Note that the main goal is not to get the correct
>> type, but rather to enforce more checks at compile time. For example:
>>
>>   GENMASK_U16(16, 0)
>>
>> will raise a build bug.
>>
>> This series is a continuation of:
>>
>>   https://lore.kernel.org/intel-xe/20240208074521.577076-1-lucas.demarchi@intel.com
>>
>> from Lucas De Marchi. Above series is one year old. I really think
>> that this was a good idea and I do not want this series to die. So I
>> am volunteering to revive it.
>>
>> Meanwhile, many changes occurred in bits.h. The most significant
>> change is that __GENMASK() was moved to the uapi headers.
>>
>> In v4 an onward, I introduce one big change: split the definition of
>> the asm and non-asm GENMASK(). I think this is controversial.
>> Especially, Yury commented that he did not want such split. So I
>> initially implemented a first draft in which both the asm and non-asm
>> version would rely on the same helper macro, i.e. adding this:
>>
>>   #define __GENMASK_t(t, w, h, l)			\
> 
> I thought we agreed on renaming...
> 
>>   	(((t)~_ULL(0) - ((t)1 << (l)) + 1) &		\
>>   	 ((t)~_ULL(0) >> (w - 1 - (h))))
>>     
>> to uapi/bits.h. And then, the different GENMASK()s would look like
>> this:
>>
>>   #define __GENMASK(h, l) __GENMASK_t(unsigned long, __BITS_PER_LONG, h, l)
> 
> Ditto.

I forgot to update the cover letter, my bad…

>> and so on.
>>     
>> I implemented it, and the final result looks quite ugly. Not only do
>> we need to manually provide the width each time, the biggest concern
>> is that adding this to the uapi is asking for trouble. Who knows how
>> people are going to use this? And once it is in the uapi, there is
>> virtually no way back.
>>
>> Finally, I do not think it makes sense to expose the fixed width
>> variants to the asm. The fixed width integers type are a C
>> concept. For asm, the long and long long variants seems sufficient.
>>
>> And so, after implementing both, the asm and non-asm split seems way
>> more clean and I think this is the best compromise. Let me know what
>> you think :)
>>
>> As requested, here are the bloat-o-meter stats:
>>
>>   $ ./scripts/bloat-o-meter vmlinux_before.o vmlinux_after.o 
>>   add/remove: 0/0 grow/shrink: 4/2 up/down: 5/-4 (1)
>>   Function                                     old     new   delta
>>   intel_psr_invalidate                         666     668      +2
>>   mst_stream_compute_config                   1652    1653      +1
>>   intel_psr_flush                              977     978      +1
>>   intel_dp_compute_link_config                1327    1328      +1
>>   cfg80211_inform_bss_data                    5109    5108      -1
>>   intel_drrs_activate                          379     376      -3
>>   Total: Before=22723481, After=22723482, chg +0.00%
>>
>> (done with GCC 12.4.1 on a defconfig)
> 
> What defconfig? x86_64_defconfig?

Yes, x86_64 defconfig.


Yours sincerely,
Vincent Mailhol